Wetting characteristics of underwater micro-patterned surfaces

Wetting phenomena of underwater systems are important for marine and other aquatic environment applications. Here, we study the underwater wetting characteristics of textured surfaces, where these surfaces are microfabricated on silicon substrates having pillar configurations. Using relevant experimental data, we have found that the equilibrium configuration of oil drops on such underwater substrates cannot be adequately characterized by the conventional wetting theories of Wenzel and Cassie-Baxter, which have been successful in validating similar wetting phenomena on pillared structures kept in ambient air. Further, we conducted contact angle hysteresis measurements for these underwater substrates and found that, except for large pillar pitch (i.e., 100 mu m), the obtained results deviate significantly from the conventional theories. We argue that the energy barrier in terms of the critical pressure associated with displacing an outer dense liquid medium (i.e., water in this case) while filling up the space between the pillars and the dynamics of liquid-liquid displacement (i.e., oil-water system in this case) are equally important for the underwater configuration of liquid drops on such textured surfaces.